Ok, as for precedent, I found a post by Apache Thunder on *that* forum in which he successfully updated a sysNAND to 9.2, but it wasn't as straightforward as just installing .CIAs, which bricked him initially. It involved injecting firm0/firm1.bins from a working 9.2 using rxTools and flashing the modified emuNAND to sysNAND. I'll try PMing him later and see if he wouldn't mind posting a little guide about his success.
well, yes, gw does weird things to the core fw, so when you try to update it it does not work.
maybe using ninjhax + fbi would be ok since it is not using gw.